Recently a quite unexpected situation has been found in which two distinct seams of conical intersection originating from the same two states—one the accidental intersection of two states of different symmetries and one the accidental intersection of two states of the same symmetry—intersect. These confluences can now be systematically identified using an algorithm that relies solely on information obtained from the symmetry-allowed seam. It is necessary to ask whether, in the absense of such an algorithm, these confluences have been overlooked in the past. In this work the well studied seam of conical intersection in is reinvestigated using the above noted algorithm with surprising results.

Numerical tests are presented for a method that combines the time-dependent self-consistent-field (TDSCF) method with the reaction path Hamiltonian (RPH) derived by Miller, Handy, and Adams [J. Chem. Phys. 72, 99 (1980)]. The theoretical basis for this TDSCF-RPH method was presented in a previous paper. The equations of motion were derived for three different cases: (1) zero coupling matrix (i.e., zero reaction path curvature and zero coupling between the normal modes); (2) zero reaction path curvature and nonzero coupling between the normal modes; and (3) zero coupling between the normal modes and nonzero but small reaction path curvature. For these three cases the dynamics can always be reduced to a one-dimensional numerical time propagation of the reaction coordinate. In this paper the TDSCF-RPH methodology for all three cases is tested by comparing the TDSCF-RPH dynamics to exact quantum dynamics based on the exact Hamiltonian for simple model systems. The remarkable agreement indicates that the TDSCF-RPH method could be useful for the calculation of the real-time quantum dynamics of a wide range of chemical reactions involving polyatomic molecules.

A linearized approximation to the semiclassical initial value representation (SC-IVR), referred to herein as the LSC-IVR, was used by us in a recent paper [J. Chem. Phys. 108, 9726 (1998)] to calculate reactive flux correlation functions for a model of a chemical reaction on a single potential energy surface. This paper shows how the LSC-IVR—which is much easier to apply than the full SC-IVR because it linearizes the phase difference between interfering classical trajectories—can be applied to electronically nonadiabatic processes, i.e., those involving transitions between different potential-energysurfaces. Applications to several model problems are presented to show its usefulness: These are the nonadiabaticscattering problems used by Tully to test surface-hopping models, and also the spin–boson model of coupled electronic states in a condensed phase environment. Though not as accurate as the full SC-IVR, the LSC-IVR does a reasonably good job for all these applications, even describing correctly Stuckelberg oscillations (interference between nonadiabatictransitions) and the transition between coherent and incoherent behavior in the spin–boson example.

The Monte Carlo method to obtain the electron-pair density for the atoms helium to neon has been applied. The wave functions of Schmidt and Moskowitz [J. Chem. Phys. 93, 4172 (1990)] to take into account the dynamic correlation among the electrons have been used. For the atoms Be, B and C we have considered the nondynamic correlation due to the near degeneracy by means of a configuration interactionwave function and for Li and Be we have also varied the central part of the wave function. A study of the differences between the correlated and the Hartree–Fock results has been carried out. Finally we have also calculated the interelectronic moments, and the value of the electron pair density at the coalescence point for all the atoms considered.

Modified virtual orbitals are proposed for multi-reference configuration interaction (MRCI) treatments and a modified Fock operator is defined for the orbital transformation. The main property of the modified orbitals is to improve the convergence properties of the configuration interaction (CI) expansion, which can be exploited to truncate, partially, the expansion in the external space. Simple tests are presented to show that the orbital transformation may be useful to perform FullCI type of treatments for subsets of orbitals and electrons, and to improve the MRCI second-order corrections and energies. Compared to other well-established techniques for accurate MRCI treatments, it is believed that this method offers advantages for electronic structures with many active orbitals and electrons using large orbital basis sets.

We extend our noniterative local correlation method [P. E. Maslen and M. Head-Gordon, Chem. Phys. Lett., 283, 102 (1998)] by defining a hierarchy of local spaces, ranging from small to large. The accuracy of the local method is then examined as a function of the size of the local space. A medium size local space recovers of the MP2 correlation energy, and reproduces fine details of the potential energy surface such as rotational barriers with an RMS error of 0.2 kcal/mol and a maximum error of 0.4 kcal/mol. A large local space recovers of the correlation energy and yields rotational barriers with a RMS error of 0.05 kcal/mol and a maximum error of 0.1 kcal/mol, at significantly increased computational cost.

The most recent methods in quantum chemical geometryoptimization use the computed energy and its first derivatives with an approximate second derivative matrix. The performance of the optimization process depends highly on the choice of the coordinate system. In most cases the optimization is carried out in a complete internal coordinate system using the derivatives computed with respect to Cartesian coordinates. The computational bottlenecks for this process are the transformation of the derivatives into the internal coordinate system, the transformation of the resulting step back to Cartesian coordinates, and the evaluation of the Newton–Raphson or rational functionoptimization (RFO) step. The corresponding systems of linear equations occur as sequences of the form where can be regarded as a perturbation of the previous symmetric matrix They are normally solved via diagonalization of symmetric real matrices requiring operations. The current study is focused on a special approach to solving these sequential systems of linear equations using a method based on the update of the inverse of the symmetric matrix For convergence, this algorithm requires a number of operations with an factor for only the first calculation. The method is generalized to include redundant (singular) systems. The application of the algorithm to coordinate transformations in large molecular geometryoptimization is discussed.

The Doppler-selected time-of-flight technique was used to study the formation of H and D in the photolysis of and its isotopomers. The combination of measurements for the photofragment kinetic energy release and the anisotropy parameter distributions allows us to differentiate, for the first time, three distinct pathways which are involved in C–H (C–D) bond fission. In conjunction with a recent ab initio theoretical investigation, the mechanisms for this complicated multichannel dissociation process are proposed. In particular, two distinct dissociation pathways are elucidated for the two-fragments channel One pathway invokes a perpendicular-type transition in absorption, which subsequently undergoes intersystem crossing to the triplet surface and then dissociates. The fragmentation via this route yields fast with a negative β parameter. Alternatively, a parallel-type excitation is involved, followed by internal conversion to the ground-statesurface on which dissociation occurs. This pathway results in less kinetic energy release and yields a positive β parameter. An intriguing isotope effect is revealed, which calls for further theoretical investigations.

The spectrum of the electronic state of jet-cooled 9-phenylfluorene– has been measured by two color resonant enhanced multiphoton ionizationspectroscopy. The cation ground states of these complexes have also been studied by mass analyzed threshold ionization (MATI) spectroscopy in a 1+1 excitation process with various intermediate states in Ab initio calculations in conjunction with the spectroscopy have determined that the phenyl ring at the 9 position is perpendicular to the plane of the fluorene moiety yielding an overall symmetry of The Ar complexes for exhibit multiple isomers which are identified in the spectrum and confirmed by MATI spectroscopy. The structure of these isomers is determined by spectral analysis and additivity rules as well as atom–atom calculations using a Lennard-Jones potential. Vibrational dynamics from selected vibronic levels are observed by the appearance of the picosecond or nanosecond time delayed MATI spectra. Vibrational redistribution and dissociation of the clusters are measured with nanosecond and picosecond time resolution. It is found that different isomers of the cluster show dramatically different rates of redistribution for several vibronic bands.

We report the first clear evidence of dissociative electron attachment involving electron capture by a Rydberg molecule from another Rydberg molecule. We observed the formation of from excimer-laser-irradiated in the presence of toluene (or benzene). Results indicate that is formed via electron capture by Rydberg states of molecules from high Rydberg states of the hydrocarbon molecules.

A symmetry-adapted filter-diagonalization method is used to calculate the vibrational spectrum of planar acetylene. In this method, vibrational eigenvalues in a given symmetry are obtained by solving a generalized eigenproblem in which the Hamiltonian and overlap matrices are assembled from symmetry-adapted correlation functions. Since no filtered state is explicitly needed, the calculation requires a relatively small memory. The numerical efficiency is further improved as the correlation functions belonging to various symmetry species are generated from a single wave packet. Comparison with existing data for the acetylene system confirms its accuracy and efficiency.

Three-dimensional potential energy surfaces for the and states of HNF are reported in the present paper. The ab initio calculations are carried out at the multireference configuration interaction (MRD–CI) level of theory employing a large basis set. The potential surface possesses a deep potential well. Both surfaces have a bent equilibrium, at approximately 100 deg for the ground state and at about 125 deg for the excited one. The two electronic states become degenerate at the linear geometry. Variational calculations for the vibrational energies and the corresponding wave functions have been performed on three-dimensional fitted potential energy surfaces. The first 101 levels of the state and the lowest 51 levels of the manifold are reported, and their vibrational modes are assigned on the basis of the nodal structure of the corresponding wave functions. The vibrational states consist of well-defined polyads with polyad quantum number where are the H–N stretching, bending, and N–F stretching quantum numbers, respectively. The calculated barrier height, vertical and adiabatic excitation energies, as well as the dissociation limits, agree satisfactorily with the available experimental data. This underlines that the overall accuracy of the potential energy surfaces is good.

When a sample is codeposited at approximately 5 K with a beam of neon atoms that have been excited in a microwavedischarge, the infrared spectrum of the resulting deposit includes prominent absorptions not only of but also of several other neutral and ionic species. The absorptions assigned to and are consistent with previous spectroscopic identifications of these species. As at lower energies, the isomer of contributes to the product spectrum. Higher level ab initio calculations of the fundamental vibrations of this isomer and of its carbon-13 substituted counterpart give improved agreement with the experimentally observed infrared spectrum and, together with the results of a supplementary experiment, provide further support for the assignment of a structured absorption near 500 cm−1 to this species, rather than to Uncharged is readily destroyed by visible radiation, with concomitant growth in the absorptions of Photodestruction of ionic species occurs in the ultraviolet spectral region. Evidence is presented for the stabilization of and of in these experiments. The latter species undergoes photodestruction in the near infrared spectral region. Two absorptions are tentatively assigned to the fragment ion.

The nature and importance of nonadditive three-body interactions in the ionic cluster have been studied by supermolecule Mo/ller–Plesset (MP) perturbation theory and coupled-cluster method, and by symmetry-adapted perturbation theory (SAPT). The convergence of the SAPT expansion was tested by comparison with the results obtained from the supermolecule Mo/ller–Plesset perturbation theory calculations through the fourth order (MP2, MP3, MP4SDQ, MP4), and the coupled-cluster calculations including single, double, and approximate triple excitations [CCSD(T)]. It is shown that the SAPT results reproduce the converged CCSD(T) results within 10%. The SAPT method has been used to analyze the three-body interactions in the clusters with water molecules located either in the first or the second solvation shell. It is shown that at the Hartree–Fock level the induction nonadditivity is dominant, but it is partly quenched by the Heitler–London and exchange-induction/deformation terms. This implies that the induction energy alone is not a reliable approximation to the Hartree–Fock nonadditive energy. At the correlated level, the most important contributions come from the induction-dispersion and the MP2 exchange energies. The exchange-dispersion and dispersion nonadditivities are much smaller, and for some geometries even negligible. This suggests that it will be difficult to approximate the three-body potential for by a simple analytical expression. The three-body energy represents only 4%–7% of the pair CCSD(T) intermolecular energy for the cluster, but can reach as much as 18% for Particular attention has been paid to the effect of the relaxation of the geometry of the subsystems.

The microwave spectrum of the NBr radical in the ground electronic state has been observed by a source modulated spectrometer. The NBr radical was generated in a free space cell by a dc glow discharge in a mixture of and He. The spectrum with three spin components of both two isotopomers, and was observed. The spectrum showed complicated splitting by the hyperfineinteractions due to both bromine and nitrogen nuclei. The molecular constants including the magnetic hyperfine and nuclear quadrupolar hyperfineinteraction constants were determined by analyzing the observed spectrum. The spin density of the unpaired electrons was estimated from the observed hyperfine coupling constants to be 73.4% and 22.4% on the nitrogen and the bromine atoms, respectively.

An ab initio investigation of the electric-field-gradient-induced birefringence of and is presented. All linear and nonlinear optical properties contributing to the induced anisotropy of the refractive index are computed by means of coupled cluster singles and doubles response theory. For systems of cubic or icosahedral symmetry, the only nonvanishing contribution to the induced birefringence within the semiclassical approach is due to the frequency-dependent dipole–dipole–quadrupole and dipole–dipole–magnetic dipole hyperpolarizabilities. These hyperpolarizabilities are important for accurate experimental determinations of the molecular quadrupole moment from electric-field-gradient-induced birefringencemeasurements.

The fluorescence excitation spectrum and the single vibronic level dispersed fluorescence spectra in the region of the transition were measured for jet-cooled 1-phenylpyrrole. The 0–0 band was observed at 35 493 cm−1. Long and low-frequency progressions with somewhat irregular intensity distributions appeared on both spectra, and were assigned to torsional motion. The torsional energy levels in the and states were obtained up to 25 and 16 quanta, respectively. The torsional potentials in both states could be determined from the sufficient number of energy levels observed. In the state the most stable conformation was determined to be a twisted form with a dihedral angle of 38.7°, where the planar barrier height was calculated to be 457 cm−1, and the perpendicular to be 748 cm−1. On the other hand, it was discovered that 1-phenylpyrrole in the state also had a twisted form with a somewhat smaller dihedral angle of 19.8°, and that the barrier to planarity was 105 cm−1 and to perpendicularity, 1526 cm−1. These facts indicated that the electronic excitation caused 1-phenylpyrrole to be rigid to twist. 1-Phenylpyrrole and its derivatives have been reported as a group of twisted intramolecular charge-transfer (TICT) molecules. No indication of TICT appeared on the shape of the -state torsional potential determined. The relation between torsional potential and TICT is discussed based on the results of this study.

The partition function, of water is calculated by explicit summation of vibration–rotation levels computed using variational nuclear motion calculations. Temperatures up to 6000 K are studied. Estimates are obtained for the heat capacity the Gibbs enthalpy factor (gef), the Helmholtz function (hcf), and the entropy of gas-phase water as a function of temperature. To get converged results at higher temperatures it is necessary to augment the accurate list of energy levels. This is done using estimates for all the vibrational band origins to dissociation and rotational levels calculated using Padé approximants. The widely used method of computing the internal partition function as the product of vibrational and rotational partition functions is tested and found to overestimate the partition function by up to 10%. The present estimates of and are probably the most accurate available for water at temperatures, above 2000 K. Errors, as a function of temperature, are estimated in each case.

The dynamics of HI photodissociation following absorption in the A band (200 to 300 nm) was investigated by Dopplerspectroscopy at the Lyman α transition of the H-atom photofragments. Measurements of both the branching ratios for the formation of spin-orbit excited and ground state atoms and the angular distributions of the recoil velocity for these two photofragment channels were obtained at nine photolysis wavelengths between 212.5 and 266 nm. These results show that ground state products result from a transition of perpendicular symmetry (ΔΩ=1), while the excited state atoms are produced from a parallel transition (ΔΩ=0). These experimental results, in combination with total absorption cross section data obtained prior to the present study, have enabled a calculation of the potential curves for the dissociative excited states. The outcome of the calculation was found to be in good qualitative agreement with the model of HI electronic structure originally suggested by Mulliken.

A Monte Carlo model has been developed which provides a very detailed picture of conditions during multiphoton infrared fragmentation experiments, as performed in an ion trap. Typically, two types of ion traps are used, an ion cyclotron resonance (ICR) instrument, and a quadrupoleion trap.Experiments fall into three separate categories: Low background gas pressure combined with either high or low intensity laser radiation, and moderate background pressures with low intensity laser radiation. Each set of experimental conditions brings to the simulation a dependency on a particular set of variables, and these can be refined to give a self-consistent picture of the complete photofragmentation process. At the low gas pressures found in ICR traps, the simulation of experiments run at low laser intensities shows that radiative decay has an important influence on photofragment yield. In the same type of trap, but at high laser intensities, pulse shape and stimulated emission become important. Finally, at pressures found in a typical quadrupoleion trap collisions with the helium background have a significant effect on the outcome of infrared excitation; however, the time scale of an experiment is such that radiative decay can also influence the results. The model has been applied to the infrared photofragmentation of the protonated diethyl ether dimer, where it successfully accounts for experimental results recorded under each of the three conditions identified above. Under circumstances where photofragmentation is in competition with either radiative or collisional relaxation, the calculations show that fragmentation requires the absorption of up to 20 photons (assumed to come from a laser, as opposed to the 12 photons necessary to match the critical energy of reaction.